The usage of mobile broadband services using cellular networks has shown a significant increase in recent years. In parallel to this growth there has been an ongoing evolution of 3G and 4G cellular networks, such as the High-Speed Packet Access (HSPA) and Long-Term Evolution (LTE) networks developed by members of the 3rd-Generation Partnership Project (3GPP) and WiMAX networks developed by members of the WiMAX Forum. These network technologies continue to be developed and improved to support ever-increasing performance requirements with regards to capacity, peak bit rates and coverage.
Operators deploying these networks are faced with a number of challenges related to such things as site and transport costs and availability, a lack of wireless spectrum, and so on. Many different techniques are considered for meeting these challenges and providing cost-efficient mobile broadband services.
One option for increasing the capacity and peak rates in 3GPP cellular networks is carrier aggregation. The principle behind carrier aggregation techniques, sometimes referred to as “multi-carrier” technology, is that a given mobile terminal (“user equipment,” or “UE,” in 3GPP terminology) can be served by multiple distinct carrier signals, at different frequencies and even in different frequency bands, at the same time. Previously it has been possible for a UE to use only one carrier at a given time. Carrier aggregation increases the maximum bit rate available to a given UE, and can also improve network capacity due to better resource utilization. Carrier aggregation was introduced for LTE in Release 10 of the 3GPP standards and for HSPA in Release 8. In the initial release, carrier aggregation was limited to two adjacent downlink carriers.
One principle adopted for carrier aggregation in 3GPP is that the UE is assigned a primary component carrier on which it receives most of the control information. Transmission of user data, on the other hand, can be performed on both the primary component carrier and on one or more secondary component carriers.
In one mode of operation in LTE networks, carrier aggregation can operate such that the UE receives uplink and downlink scheduling commands only on the primary component. In other modes of operation in LTE networks, the UE receives scheduling commands on all component carriers. However, regardless of the mode of operation, the UE is only required to read the broadcast channel (in order to acquire system information parameters) on the primary component carrier. System information related to the secondary component carriers can be provided to the UE in dedicated RRC messages. (For details on carrier aggregation, see 3GPP TS 36.300, v. 10.3.0, April 2011, available at http://www.3gpp.org/ftp/Specs/html-info/36300.htm.)
From the point of view of a base station (a “NodeB” or “NB” in HSPA systems, or an “evolved NodeB” or “eNB” in LTE systems), each carrier is associated with its own cell. So, support for carrier aggregation means that the base station (which may be at a single site or split among several transmission points) supports multiple cells. In other words, the base station broadcasts multiple Cell IDs.
Another option available to the operator is the deployment of home base stations or other small base stations that complement the traditional macro cellular network. In LTE, home base stations are known as “HeNBs,” while in HSPA systems these home base stations are called “HNBs”. The Femto Forum (www.femtoforum.org) refers to these small, complementary base stations as “femtocells” or simply “femtos.” Some of the benefits of these small base stations are lower site costs, due to smaller physical size and lower output power, as well as increased system capacity and coverage due to the closer deployments of base stations to the end user.
A network operator can configure cells as Open, Hybrid or Closed. Open cells are open to use for all subscribers, with no preference to perform cell reselection to individual cells. Closed cells broadcast a CSG (Closed Subscriber Group) cell type indicator and a CSG identifier. The broadcasted information elements (IEs) are called “CSG Indicator” and “CSD ID”, the former indicating values of either “true” or “false” and the latter indicating a 27-bit identifier uniquely pointing at a CSG in the used Public Land Mobile Network (PLMN). Closed cells are only available for use by mobiles belonging to the identified CSG. When the cell is closed, the CSG Indication broadcasted has the value “true”. Hybrid cells also broadcast a CSG (Closed Subscriber Group) identity, but in this case the CSG Indication broadcasted has the value “false”. Hybrid cells are available for all users. In addition, users belonging to the CSG have a preference for selecting CSG cells with the same CSG identity.
The number of deployed home base stations could be very large. For that reason, and because they are considered to be less reliable nodes, solutions have been introduced for home base stations to connect to the core network via a home base station gateway. For LTE, this home base station gateway is referred to as HeNB GW; for HSPA, it is known as HNB GW. For the purposes of this document, the term H(e)NB GW will be used to refer to either.
The H(e)NB GW serves to hide the home base station from the rest of the network. In the LTE/SAE case, the HeNB GW is optional. As a result, the S1 interface is used by the HeNB to connect to either an HeNB or the core network (the evolved packet core, or EPC, in LTE), and the HeNB GW therefore has S1-interfaces on both sides of it. To the rest of the network, an HeNB GW looks like a large eNB with many cells. From an HeNB's point of view, a HeNB GW looks like a core network node (i.e, the Mobility Management Entity, or MME).
The LTE architecture for HeNBs is illustrated in FIG. 1. In the upper part of FIG. 1, it can be seen that the illustrated HeNB 110 sends and receives control information to and from EPC 130 via the S1-MME interface. In this case, the control information is relayed via the HeNB GW 120, which appears to HeNB 110 as an MME. HeNB 110 sends and receives user data over the S1-U interface; this is via the SeGW 140, which is another component of EPC 130. Still another component of the EPC 130 is the Home eNodeB Management System (HeMS), which facilitates operation and maintenance (OAM) of HeNBs.
In the lower part of FIG. 1, several HeNBs 110 are connected to MMEs in the EPC 130 via HeNB GW 120, which is considered part of the radio access network (RAN). However, another HeNB 110 is connected directly to the MME/S-GW functionalities 170 in EPC 130, in the same manner as conventional eNBs 160.
An HeNB 110 that is connected to the network via an HeNB GW 120 is connected to only one HeNB GW 120. In this configuration, the HeNB 110 does not support the network node selection (NAS Node Selection Function, or NNSF). Instead, the HeNB GW 120 supports the network node selection functionality, enabling support for MME-pools. In the case when the HeNB 110 connects directly to the EPC 130, the HeNB 110 supports the network node selection functionality.
In the HSPA/WCDMA case, an HNB GW is mandatory. A new Iuh interface is defined between HNBs and the HNB GW, and the normal Iu interface is used between the HNB GW and the core network. To the rest of the network, the HNB GW just looks like a large RNC with many service areas. (Service area is the UTRAN concept for one or multiple cells). The HNB only connects to one HNB GW, so the HNB does not have the network node selection functionality. Instead, the HNB GW supports the network node selection functionality enabling support for MSC and SGSN-pools.
According to the current standards, each logical H(e)NB only supports a single cell. In HSPA/WCDMA this is a specified restriction aimed at reduce HNB complexity. In LTE this is due to the use of all of the 28 bits of E-UTRAN Cell ID (E-CGI) for routing of S1 signaling towards the HeNBs. For normal eNBs the first 20 bits of the E-CGI correspond to the eNB ID used for message routing and the remaining 8 bits correspond to the Cell ID within that eNB. For HeNBs, the entire 28 bits of the E-CGI indicate a specific HeNB and are used for message routing.